Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate

As is well known, the heat transfer coefficient of a solar air heater duct can be increased by providing artificial roughness on the heated wall (i.e. the absorber plate). Experiments were performed to collect heat transfer and friction data for forced convection flow of air in solar air heater rectangular duct with one broad wall roughened by wedge shaped transverse integral ribs. The experiment encompassed the Reynolds number range from 3000 to 18000; relative roughness height 0.015 to 0.033; the relative roughness pitch 60.17φ^−1.0264<p/e<12.12; and rib wedge angle (φ) of 8, 10, 12 and 15°. The effect of parameters on the heat transfer coefficient and friction factor are compared with the result of smooth duct under similar flow conditions. Statistical correlations for the Nusselt number and friction factor have been developed in terms of geometrical parameters of the roughness elements and the flow Reynolds number.     

Natural convection in a triangular cavity filled with air under the effect of external air stream cooling

Natural convection in a triangular cavity filled with air is investigated numerically. In this paper, the cavity is exposed to air stream cooling exerted on its sides and it is heated by a fixed heat flux from the base. The air inside the cavity is assumed to be laminar and obeying Boussinesq approximation. The governing equations are solved numerically using the finite volume technique with SIMPLE algorithm. The results are achieved with a range of Rayleigh number (10⁴ < Ra < 10⁷), free stream Reynolds number (10³ < Re_∞ < 1.5 × 10⁴), four aspect ratios (AR = 0.25, 0.5, 0.866, and 1) and five inclination angles (? = 0°, 30°, 45°, 60°, 90°). The influence of these parameters is displayed on the stream function, isotherms lines, local and average Nusselt numbers. The results reveal that the heat transfer rate increases as Rayleigh number, free stream Reynolds number and AR increase. The highest heat transfer rate is obtained at ? = 0° while the lowest one is obtained at ? = 90°. Furthermore, as the AR augments, the local and average Nusselt numbers are enhanced and the stream function is formed of two symmetric counter‐rotating vortices.     

Analysis of heat transfer augmentation and flow characteristics due to rib roughness over absorber plate of a solar air heater

A computational analysis of heat transfer augmentation and flow characteristics due to artificial roughness in the form of ribs on a broad, heated wall of a rectangular duct for turbulent flow (Reynolds number range 3000–20,000, which is relevant in solar air heater) has been carried out. Shear stress transport k−ω turbulence model is selected by comparing the predictions of different turbulence models with experimental results available in the literature. A detailed analysis of heat transfer variation within inter rib region is done by using the selected turbulence model. The analysis shows that peak in local heat transfer coefficient occurs at the point of reattachment of the separated flow as observed experimentally. The results predict a significant enhancement of heat transfer in comparison to that for a smooth surface. There is a good matching between the predictions by SST k−ω and experimental results. In this work, nine different shapes of rib are examined using SST k−ω model and compared on the basis of heat transfer enhancement, friction characteristics and performance index considering heat transfer enhancement with the same pumping power.     

Experimental investigations on overall cooling effect of ribbed channel with air bleeds

An experimental investigation on overall heat transfer performance of a rectangular channel, in which one wall has periodically placed oblique ribs to enhance heat exchange and cylindrical film holes to bleed cooling air, has been carried out in a hot wind tunnel at different mainstream temperatures, hot mainstream Reynolds numbers, coolant Reynolds numbers and blowing ratios. To describe the cooling effect of combined external coolant film with the internal heat convection enhanced by the ribs, the overall cooling effectiveness at the surface exposed in the mainstream with high temperature was calculated by the surface temperatures measured with an infrared thermal imaging system. The total mass flow rate of cooling air through the coolant channel was regulated by a digital mass flow rate controller, and the blowing ratio passing through the total film holes was calculated based on the measurements of another digital-type mass flow meter. The detailed distributions of overall cooling effectiveness show distinctive peaks in heat transfer levels near the film holes, remarkable inner convective heat transfer effect over entire channel surface, and visible conductive heat transfer effect through the channel wall; but only when the coolant Reynolds number is large enough, the oblique rib effect can be detected from the overall cooling effectiveness; and the oblique bleeding hole effect shows the more obvious trend with increasing blowing ratios. Based on the experimental data, the overall cooling effectiveness is correlated as the functions of Re_m (Reynolds number of hot mainstream) and Re_c (Reynolds number of internal coolant flow at entrance) for the parametric conditions examined.     

Optimized electrode arrangement in solar air heater

Laminar forced convection inside the solar air heater with various wire electrode arrangements are numerically examined for heat transfer enhancement using electrohydrodynamic technique. The electric field is generated by the wire electrodes charged with DC high voltage ranging from 7.5 to 17.5 kV. Reynolds number corresponding to the flow considered is between 100 and 2000. The numerical modeling of computational fluid dynamics includes the interactions among electric field, flow field, and temperature field. It is found that the enhancement of heat transfer coefficient with the presence of electric field increases in relation with the supplied voltage but decreases when the Reynolds number and the distance between electrode and channel surface are augmented. The optimized electrode arrangement, which obtains the best heat transfer enhancement is investigated incorporating with the pressure drop consideration. The heat transfer enhancement is also depended on the number of electrodes per length and the channel dimension.     

Forced convection heat transfer from a circular cylinder in crossflow to air and liquids

Local and average heat transfer by forced convection from a circular cylinder is studied for Reynolds number from 2 × 10³ to 9 × 10⁴ and Prandtl number from 0.7 to 176. For subcritical flow, the local heat transfer measurement indicates three regions of flow around the cylinder: laminar boundary layer region, reattachment of shear layer region and periodic vortex flow region. The average heat transfer in each region is calculated and correlated with the Reynolds number and the Prandtl number. The Nusselt number in each region strongly depends on the Reynolds number and the Prandtl number with different power indices. An empirical correlation for predicting the overall heat transfer from the cylinder is developed from the contributions of heat transfer in these three regions.     

Three-dimensional numerical simulation of the slip flow through triangular microchannels

Three-dimensional numerical analysis for fully developed incompressible fluid flow and heat transfer through triangular microchannels over the slip flow regime is simulated in this paper. In order to study the flow through the channel, the Navier–Stokes equations are solved in conjunction with slip/jump boundary conditions. The influences of Knudsen number (0.001 < Kn < 0.1), aspect ratio (0.2 < A < 4.5), and Reynolds number (1 < Re < 15) on the fluid flow and heat transfer characteristics are extensively investigated in the paper. The numerical results reveal that the rarefaction decreases the Poiseuille number, while its effect on the Nusselt number completely depends on the interaction between velocity slip and temperature jump. It is also found that the aspect ratio has an important role in the analysis, but the variation of Reynolds number is less remarkable.     

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.     

Comparative study between parallel and counter flow configurations between air and falling film desiccant in the presence of nanoparticle suspensions

A comparative numerical study is employed to investigate the heat and mass transfer between air and falling film desiccant in parallel and counter flow configurations. Nanoparticles suspensions are added to the falling film desiccant to study heat and mass transfer enhancements. The numerical results show that the parallel flow channel provides better dehumidification and cooling processes of the air than counter flow configuration for a wide range of pertinent parameters. Low air Reynolds number enhances the dehumidification and cooling rates of the air and high air Reynolds number improves the regeneration rate of the liquid desiccant. An increase in the channel height results in enhancing the dehumidification and cooling processes of air and regeneration rate of liquid desiccant. The dehumidification and cooling rates of air are improved with an increase in the volume fraction of nanoparticles and dispersion factor. Copyright © 2003 John Wiley & Sons, Ltd.     

Correlations for solar air heater duct with V-shaped perforated baffles as roughness elements on absorber plate

An experimental investigation has been carried out to study the effect of heat transfer and friction characteristics of air passing through a rectangular duct which is roughened by V-down perforated baffles. The experiment encompassed Reynolds number (Re) from 3800 to 19,000, relative roughness height (e/H) values of 0.285–0.6, relative roughness pitch (P/e) range of 1–4 and open area ratio values from 12% to 44%. The effect of roughness parameters on Nusselt number (Nu) and friction factor (f) has been determined and increase in heat transfer and friction loss has been observed for ducts having a roughened test plate. Maximum Nusselt number is observed for the relative roughness pitch ranging from 1.5 to 3 for flow and geometrical parameters under consideration. The experimental data have been used to develop Nusselt number and friction factor correlations as a function of roughness and flow parameters.     

Augmentation of heat transfer coefficient by using 90° broken transverse ribs on absorber plate of solar air heater

An experimental investigation has been carried out to study the heat transfer coefficient by using 90° broken transverse ribs on absorber plate of a solar air heater; the roughened wall being heated while the remaining three walls are insulated. The roughened wall has roughness with pitch (P), ranging from 10–30 mm, height of the rib of 1.5 mm and duct aspect ratio of 8. The air flow rate corresponds to Reynolds number between 3000–12,000. The heat transfer results have been compared with those for smooth ducts under similar flow and thermal boundary condition to determine the thermal efficiency of solar air heater.     

Heat transfer augmentation for air jet impinged on a rough surface

An experimental investigation of heat transfer from a round air jet impinging normally from below onto a heated square plate was performed. The objective of the investigation was to study the effect of roughness on both the heat transfer and the fluid flow characteristics. Smooth and rough plates were, therefore, used in the course of the experiments. The heat transfer data were collected for four jet Reynolds numbers, ranging from 6500 to 19 000. The Reynolds numbers are based on the jet-exit velocity (U_e) and the nozzle-exit diameter (D), Re_e=U_eD/ν. The nozzle-to-plate distance ranged from 0.05 to 15 nozzle-exit diameter to cover both the potential core and the far regions of the jet flow. The roughness was composed of cubes of 1 mm dimension distributed uniformly along the plate. The local and average Nusselt number values for the rough plate showed an increase ranging from 8.9% to 28% over those for the smooth plate. Roughness was found to have a strong effect on the flow characteristics; it affected the mean velocity as well as the turbulence intensity of the flow. The mean velocity profiles for the smooth case at radial distances of r/D=1 and r/D=2.5 showed steeper near-wall velocity gradients compared with the profiles of the rough case, where r is the radial distance measured from the plate center along the plate centerline. In addition, roughness caused an increase in the turbulence intensity of the flow.     

Investigation of thermal performance of solar air heater having roughness elements as a combination of inclined and transverse ribs on the absorber plate

An experimental investigation has been carried out to study the heat transfer and friction characteristics by using a combination of inclined as well as transverse ribs on the absorber plate of a solar air heater. The experimental investigation encompassed the Reynolds number (Re) ranges from 2000 to 14 000, relative roughness pitch (p/e) 3–8 and relative roughness height (e/D_h) 0.030. The effect of these parameters on the heat transfer coefficient and friction factor has been discussed in the present paper and correlations for Nusselt number and friction factor has been developed within the reasonable limits. A procedure to compute the thermal efficiency based on heat transfer processes in the system is also given and the effect of these parameters on thermal efficiency has been discussed.     

Experimental study of heat transfer coefficient with rectangular baffle fin of solar air heater

This paper presents an experimental analysis of a single pass solar air collector with, and without using baffle fin. The heat transfer coefficient between the absorber plate and air can be considerably increased by using artificial roughness on the bottom plate and under the absorber plate of a solar air heater duct. An experimental study has been conducted to investigate the effect of roughness and operating parameters on heat transfer. The investigation has covered the range of Reynolds number Re from 1259 to 2517 depending on types of the configuration of the solar collectors. Based on the experimental data, values of Nusselt number Nu have been determined for different values of configurations and operating parameters. To determine the enhancement in heat transfer and increment in thermal efficiency, the values of Nusselt have been compared with those of smooth duct under similar flow conditions.     

Experimental Investigation of the Effect of Tube-to-Tube Porous Medium Interconnectors on the Thermohydraulics of Confined Tube Banks

This experimental study investigates the effect of tube-to-tube copper porous interconnectors on the thermohydraulic performance of an in-line and staggered confined tube bank. The porous medium, having a transverse thickness equal to that of the diameter of the tube (9 mm), connects longitudinally six successive tubes kept as in-line and staggered arrangements with a square pitch of 2.0. The tubes are subjected to a constant and uniform heat flux and are cooled by forced convection under laminar-transition flow range (200 < Reynolds number < 1500) using air with a Prandtl number of 0.71 as cooling fluid. Experimental data presented here establish that by introducing tube-to-tube porous medium interconnectors for the maximum Reynolds number tested here, a reduction in the pressure drop by 18% is observed in the in-line configuration while the heat transfer is enhanced by 100% in the staggered configuration, when compared to their respective configurations without the porous medium. Defining an overall energy gain as the ratio of the heat transfer enhancement due to the presence of the porous inserts to the pressure drop incurred, it is seen that fixing the porous inserts in the in-line configuration is advantageous.     

Fuzzy modeling of the forced convection heat transfer from a V‐shaped plate exposed to an air slot jet

This paper focuses on the application of fuzzy logic (FL) to predict the forced convection heat transfer from V‐shaped plate internal surfaces exposed to an air impingement slot jet. The aim of the present paper is to consider the effects of the angle of a V‐shaped plate (Φ), slot‐to‐plate spacing ratio (Z/W), and Reynolds number (Re) variation on average heat transfer from the V‐shaped plate internal surfaces. The data used for developing the FL structure was obtained experimentally by a Mach‐Zehnder interferometer. The proposed FL was developed using MATLAB functions. It was observed that the average Nusselt number will be decreased with an increase in jet spacing and be increased with an increase in Reynolds number and angle of V‐shaped plate. Moreover, it is also shown that fuzzy logic is a powerful technique to use for predicting heat transfer due to its low error rate. The average error of the fuzzy predictions compared with experimental data was found to be 0.33% for this study. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21009     

Experimental study of the influence of collector height on the steady state performance of a passive solar air heater

Passive solar air heaters, such as solar chimneys and Trombe Walls, rely on solar-induced buoyancy-driven (natural) convection to produce the flow of air. Although buoyancy-driven convection is well understood for a single vertical plate, buoyancy-driven convection in an asymmetrically-heated channel is more problematic, and in particular, the effects of the channel height on the flow rate and heat transfer. This paper reports on experiments on test rigs resembling lightweight passive solar air-heating collectors. The test rigs were of heights 0.5, 1.0 and 2.0 m, with adjustable channel depths (20-150 mm) and heat inputs (up to 1000 W/m²). Measurements were made of the air, plate and cover temperatures, and air velocities. Results are presented as dimensionless correlations of mass flow (as Reynolds number) and efficiency against heat input (as Rayleigh number), channel depth and height. Thermal efficiency is shown to be a function of the heat input and the system height, but not of the channel depth; mass flow is shown to be a dependent on all three parameters.     

Effect of tabs on impinging heat transfer

The present work experimentally investigates the effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the fluid flow and heat transfer characteristics of an axisymmetric impinging air jet in the subcritical Reynolds number range. With this comes the expectation of a large eddy structure variation and the possibility of active control. Local heat transfer and static pressure were measured on a target plate for a round air jet issuing from a circular nozzle with rectangular tabs whose numbers and lengths changed at a constant nozzle‐to‐plate gap (L/d = 8) and jet Reynolds number (Re = 34,000). The main results are the following: When two tabs were set at the exit of the circular nozzle, Cpw and Nu profiles flatten in the direction of the tab setting. In the case of three tabs, however, among both Cpw and Nu profiles a concentric profile is found, as well as in the case without any tabs. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 561–570, 2001     

Comparative Study of Convective Heat Transfer Performance of Steam and Air Flow in Rib Roughened Channels

A comparative experimental study of heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted by using an infrared camera. Effects of Reynolds numbers and rib angles on the steam and air convective heat transfer have been obtained and compared with each other for the Reynolds number from about 4,000 to 15,000. For all the ribbed channels the rib pitch to height ratio(p/e) is 10, and the rib height to the channel hydraulic diameter ratio is 0.078, while the rib angles are varied from 90° to 45°.Based on experimental results, it can be found that, even though the heat transfer distributions of steam and air flow in the ribbed channels are similar to each other, the steam flow can obtain higher convective heat transfer enhancement capability, and the heat transfer enhancement of both the steam and air becomes greater with the rib angle deceasing from 90° to 45°. At Reynolds number of about 12,000, the area-averaged Nusselt numbers of the steam flow is about 13.9%, 14.2%, 19.9% and 23.9% higher than those of the air flow for the rib angles of 90°,75°, 60° and 45° respectively. With the experimental results the correlations for Nusselt number in terms of Reynolds number and rib angle for the steam and air flow in the ribbed channels were developed respectively.     

On the combined influence of Reynolds number and cylinder spacing for flow around a row of heated square cylinders

The characteristics of forced convection heat transfer across a row of heated square cylinders kept in side-by-side arrangement are numerically investigated to examine the combined effects of Reynolds number and cylinder spacing for Ri = 0, 60 ≤ Re ≤ 160, Pr = .71, and s/d = 1.0–8.0, where the space between cylinder surfaces is s and the cylinder size is d. A numerical study was carried out using the thermal lattice Boltzmann method. The goal of this work is to explore the transitions in heat transfer phenomenon that occurs behind the cylinder and to report the corresponding regimes for heat transfer namely synchronous, quasiperiodic, and chaotic. The proposed regime of heat flow is a function of Reynolds number and spacing. The synchronous heat regime is obtained for s/d ≥ 5.0 and quasiperiodic, chaotic regimes are observed for 3.0 ≤ s/d < 5.0, s/d < 3.0, respectively at Re = 100. The instantaneous isotherms, the power spectra of the corresponding Nusselt number signals, and the significance of cylinder Nusselt number frequency are used to examine these heat flow regimes. The heat transfer regimes for a row of heated cylinders and flow regimes for a row of unheated cylinders both have comparable appearances except for the fact that the heat transfer regime is synchronous at s/d ≥ 5.0 and flow is synchronous at s/d ≥ 4.0. The chaotic or quasiperiodic heat transfer regimes occur due to merging and strong interactions between thermal blobs shed from the cylinders. Heat transfer is synchronous at a higher spacing and characterized by independent thermal blobs shedded from the cylinders. It is reported that as spacing reduces and Reynolds number increases, the mean value of the Nusselt number experienced by all cylinders increases. The important outcome of the present numerical work is that for understanding heat transfer from bluff body, the transitions that occur in heat transfer are useful.